The present invention is directed to a method of determining a ground-fault current in a multi-phase system by performing a vectorial summation of the output signals of each current transformer allocated to each phase of the system and of a neutral conductor.
European Published Patent Application 0 407 310 B1 requires the use of current transformers having great accuracy. Spurious tripping can occur if this requirement is not satisfied. For instance, if the ground-fault trip is to occur at 20% of the nominal current and increases by 5% when there is a fault in the transformer, then the measured ground current at 4-times nominal phase current has a fault of 100%. According to this reference, spurious tripping can only be avoided by employing current transformers that have great accuracy, but such transformers are heavy and require more space for installation. Space is limited because modern circuit breakers also place great value on compact dimensions.
Other methods for determining a ground fault current comprise specific features in terms of the requirements placed on the current transformers. In particular, ground currents can be measured directly, and this can be done by a ground transformer in the transformer neutral point or by a summation current transformer; this method is shown in U.S. Pat. No. 5,195,009. An obstacle to the high accuracy of such measuring methods can be increased complexity or conflicts with the system configuration.